1. Field of the Invention
This invention relates generally to the fabrication of ceramic objects. In particular, this invention relates generally to forming of ceramic materials. More particularly, this invention relates to forming of high temperature ceramic superconductors and composites thereof.
2. Discussion of the Background
Recently superconductivity in polycrystalline Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 above 90.degree. K was discovered. This compound has a structure which is a modification of the cubic perovskite structure. The perovskite structure has cubic symmetry, with one metal ion at the center of each cubic unit cell and three oxygen atoms for each unit cell. The crystal structure of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7, hereinafter called the 123 structure represents a tripling of the perovskite unit cell with Y or Ba atoms at the center of each cube of the perovskite sub cells. Many materials have the 123 structure, and these materials will be referred to hereinafter as 123 material. The 123 structure is also deficient in oxygen, as compared with the perovskite structure, which would have nine oxygens instead of the seven present in fully oxygenated 123 material. The oxygen content of 123 material can be lowered to 6 while maintaining the cation structure. Lowering the oxygen content changes 123 materials from conducting, to insulating. In addition, the fully oxygenated 123 material (with oxygen content between 6.9 and 7.0) has an orthorhombic distortion of the tripled perovskite lattice unit cells, while the insulating 123 material (with oxygen content below 6.5) has a tetragonal distortion of the tripled perovskite lattice unit cells. Y can be replaced by any trivalent rare earth or lanthanum without significantly affecting the 123 structure, or the superconducting properties. In addition, the rare earths and La are so similar to Y that the cohesive properties, and phase diagrams which are determined by the cohesive properties, of materials containing Y, La or a rare earth, are all very similar.
Therefore, use of yttrium in the rest of this patent is exemplary only, since La and rare earth elements provide very similar properties, and the variation of these properties between yttrium compounds, and the homologous compounds are well known. In particular, 123 materials in which Y is replaced by similar elements, such as La, Pr, Nd, Sm, Gd, Dy, Ho, Er, or Tm, are all superconducting at over 90.degree. K.
123 superconductor applications are generally split into thin film and bulk applications. Thin film devices are relatively easy to make because no mechanical strain is involved in their fabrication. In contrast, bulk components of 123 material must either be cast, or formed. 123 material suffers from brittleness, poor ductility, oxygen instability upon heating, and phase instability above the melting point. In this regard, Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7, BaCuO.sub.2 and CuO form an eutectic at 900.degree. centigrade (C), so that above 900.degree. C. liquid and solid phases coexist, unless the heated material is essentially perfectly stoichiometric Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7. These properties provide several drawbacks to both casting and forming.
Several techniques for dealing with these problems have been attempted, and their results have been reported in the scientific literature. However, each of these methods has disadvantages. Ceramic superconductor shapes have been prepared by compacting the powders into a mold followed by a heat treatment. This type of process is reported, for example, in European Patent Application number 88201568.8 published 01.02.89, "Method of preparing an oxidic superconductor material". This publication also discloses a method for preparing submicron size particles of 123 superconductor.
Complex shapes cannot be obtained by compacting powders, because volume shrinkage occurs during the subsequent sintering steps, and results in incorrect shapes. Subsequent machining of ceramic bodies is very difficult, because the ceramics are hard and brittle, and often leads to cracks in the ceramic, and can quickly wear out machine tool bits. Because of the difficulty of machining these materials, complex shapes cannot be obtained by this method.
Powdered compacts of ceramic superconductors contained in metallic sheaths have been formed into shapes by rolling or drawing of the sheath material, as reported in "HOT-PRESSING OF YTTRIUM, BARIUM, COPPER OXIDE (Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.Y) CERAMIC POWDER" by Nishida, et al, published in Mem. Fac. Eng. Des., Kyoto Institute of Technol. 1988, 37, 96-104 (Eng). This article discloses hot extrusion of a metallic can containing superconducting YBa.sub.2 Cu.sub.3 O.sub.y ceramic powder. The can is first heated to 700.degree. to 1,000.degree. C. and then immediately deformed by hot processing, and also rapidly cooled to room temperature. The resulting material changed to the tetragonal insulating structure from the orthorhombic superconducting structure during the processing.
Rolling or drawing techniques have the disadvantage that they require a sheath. Because of the sheath, heat treatment of the formed shapes leads to shrinkage of the powder compact and results in separation of the powder from the sheath. Also, the sheath prevents oxygen from penetrating into the superconducting material during oxygen annealing. Large cracks are likely to form because of these problems, since the 123 material changes density upon sintering and oxygenation, and these cracks cannot be removed by heat treatment. In addition, this method is only suitable for preparation of wire or bar.
FIG. 8A displays a bulk object having the 123 structure before and FIG. 8B after being compressed by a process of the prior art. A bulk structure of 123 superconductor 50, is deformed to a limited amount by stress applied in the direction indicated by the arrows 51. Further deformation leads to crack formation in the deformed object 52 in FIG. 8B, crack growth and finally the deformed structure 52 is fractured, as shown by fracture lines 53.
The article by Kaibyshev et al "SUPER-PLASTICITY OF AN YTTRIUM, BARIUM, COPPER OXIDE (YBa.sub.2 Cu.sub.3 O.sub.7-x) CERAMIC COMPOUND" (Inst. Probl. Sverkhtverd. Met., Ufa, USSR); Dokl. Akad. Nauk SSSR 1989, 305 (5), 1120-30 [Tech. Phys.] (Russ), discloses superplasticity of 123 ceramic compounds between 900.degree. and 950.degree. C. after "dynamic recrystallization". As noted earlier, 900.degree. C. is the liquid phase formation temperature of YBa.sub.2 Cu.sub.3 O.sub.7 in the presence of impurity phases of BaCuO.sub.2 and CuO. Plastic deformation at high temperatures close to the melting point of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 and above the liquid phase formation temperature is easily obtained, because the diffusion rate is very high, and the liquid phase that exists between the grains allows easy movement of the grains relative to one another.
Superplasticity is defined in "Metals Handbook" published by the American Society for Metals, Metals Park Ohio, 1985, as the ability of certain metals to undergo unusually large plastic deformation without local necking or failure. This definition also applies to ceramics. "Unusually long" depends upon the material. For ceramics, less than 10% elongation is typically considered normal plastic deformation, as opposed to superplastic deformation.
The deformation behavior of YBa.sub.2 Cu.sub.3 O.sub.7 at temperatures above 900.degree. C. where liquid phase exists, and below 900.degree. C. where no liquid phases exists are very different, because below 900.degree. C. the mechanism relieving stress due to a liquid phase is absent. Therefore, below 900.degree. C. stress can only be relieved by solid state mechanisms, which are inconsequential above 900.degree. C. The temperature scale is also important because it is technologically easier to work at lower temperatures, such as below 900.degree. C., than at temperatures above 900.degree. C.
The Japanese patent to Matsumoto (JP 1-93301 (A)) describes a plastic processing method for superconductive ceramics. The critical conditions specified in this patent are a temperature range of 500.degree. to 850.degree. C. and strain rates of less than 2.2.times.10.sup.-1 per second at 850.degree. C., and 3.3.times.10.sup.-5 per second at 500.degree. C. JP 1-93301 does not mention the grain size of the material used.
The material used in this reference was sintered at between 950.degree. and 1,300.degree. C. This temperature range extends above the liquid phase formation temperature of 900.degree. C. Sintering above the liquid phase formation temperature dramatically enhances the grain growth rate. As a result of such high temperature sintering, small grain sizes, such as those disclosed hereinafter in the present invention, cannot be obtained.
JP 1-93301 also discloses a flow stress versus temperature plot (FIG. 2) which does not follow the established exponential function for the high temperature deformation of ceramics. It is probable that the results from JP 1-93301 are due to a multi-phase material, since the reported temperature range for sintering exceeds the 123 phase stability temperature of 1,015.degree. C. for Y.sub.1 B.sub.2 Cu.sub.3 O.sub.7, which results in production of a multi-phase material.
While the aforementioned teachings are useful for providing formed shapes, they do not provide a method for providing complicated shapes of 123 superconductors and composites thereof without cracks and to precise tolerances.